To understand epidural and subdural hematomas — two serious consequences of head injuries — we need to know the basic anatomy of the brain and its coverings. Imagine an evil carpenter with an electric drill intent on drilling into a person’s brain. What layers would the drill encounter in its passage from the outside of the head to its destination?
The drill would pass through the skin and then the skull (braincase) before penetrating a series of three membranes comprising the meninges. In sequence, the three membranes are the dura mater (Latin for “tough mother”), the arachnoid mater (cobwebby mother) and the pia mater (tender mother) and then finally the brain itself.
Epidural and subdural hematomas are alike in that they are masses of clotted blood (hematomas) caused by head trauma and deposited outside the brain but inside the skull. However, they differ in their locations relative to the dura mater. An epidural hematoma lies outside (on top of) the dura mater, while a subdural hematoma lies inside (beneath) the dura mater and outside the arachnoid mater. Thus, the locations of the two kinds of hematoma are encoded in their names — “epi” is Greek for “upon” and “sub” is Latin for “below.” A third kind of hematoma caused by head injuries is traumatic intracerebral hemorrhage. These occur within the brain tissue itself and are no less serious than those outside the brain, but are not the subject of the current essay.
Epidural and subdural hematomas are produced by ruptures of different blood vessels. Epidural hematomas are usually caused by bleeding from an artery that nourishes the meninges known as the middle meningeal artery, while subdural hematomas are usually due to bleeding from veins that drain blood away from the surface of the brain.
Yet another difference between epidural and subdural hematomas is what they look like on computed tomographic (CT) scans. When the bleeding was recent, both show up as intensely bright objects on the scan, but the shapes of the blood clots are different. In epidural hematomas the blood is more limited in its spread because it has to push harder to move outward in the tight space between the inner surface of the skull and the outer surface of the dura mater. In contrast, the bleeding that produces subdural hematomas is more free to spread in the looser space beneath the dura mater and typically runs from the front of the head to the rear.
One issue that applies to both kinds of hematomas is that they occupy space — sometimes a lot of it — within the braincase where there isn’t a lot of extra space to go around. As they expand they compress the brain tissue next to them and additionally raise the pressure within the skull which can damage the rest of the brain. Moreover, the hematoma is not necessarily the only problem caused by the head injury. The blow to the head that caused the bleed can also damage the brain tissue directly.
Who gets epidural hematomas? They usually occur in people with obvious and significant blows to the head, as from motor vehicle accidents. In one study they were present in 10% of head-injured patients who arrived at an emergency department in coma, but they can also be seen in conscious patients. Epidural hematomas usually occur in conjunction with skull fractures, and this is no coincidence, as the ruptured blood vessel often lies beneath the fracture. The presence of an epidural hematoma signifies a highly dangerous condition. Between 5 and 43% of people who have them die. Emergency surgery to remove the clot is the usual treatment.
When considering subdural hematomas, it is useful to divide them into acute and chronic varieties, with “acute” meaning the hematoma is new, and “chronic” meaning it has been present for at least three weeks. (The hematoma can also pass through a “subacute” phase, meaning that it has been present for 3 days to 3 weeks.) By the time an acute subdural hematoma has become chronic, it is a thick liquid instead of a solid blood clot, and also appear darker on CT scans.
Acute subdural hematomas usually occur in people with obvious and significant blows to the head. In one study they were present in 24% of the patients who arrived at an emergency department in coma, but can be present in non-comatose patients as well. Acute subdural hematomas are associated with a death rate between 30 and 90%, with a figure of 60% typically cited. Emergency surgery is the usual treatment, though studies have shown that alert patients with small subdural hematomas can do as well without surgery if monitored closely for signs of worsening.
Infants are also vulnerable to acute subdural hematomas. Neurosurgeons at the Kaohsiung Medical University in Taiwan reviewed records on 21 children, ages 6 days to 12 months, who had acute subdural hematomas. In this case series, “shaken baby syndrome” was the most common cause. Eight of the infants underwent an immediate operation, and another 11 required delayed surgery. While most of the children did well, one baby died and another 7 sustained moderate to severe disabilities from their injuries.
Chronic subdural hematomas often show up in patients over 60 years of age in whom the head injuries that caused them might have seemed trivial when they occurred, or might even have been forgotten. Older people are especially vulnerable due to the fact that their brains have atrophied (shrunk) and the veins draining the surface of the brain are stretched and fragile, easily disrupted by glancing blows. Risk of subdural hematoma rises still higher if the individual falls a lot, drinks alcohol a lot or takes blood-thinning medication.
Subdural hematomas can expand progressively to the point of causing symptoms like headache, slurred speech, confusion, lethargy, unsteadiness or even a seizure. Surgery to remove the hematoma and stop the bleeding is the typical treatment, and 93 to 97% of patients survive to 30 days after surgery. Most regain their pre-injury level of function. Milder cases of chronic subdural hematoma can be monitored without surgery.
(C) 2006 by Gary Cordingley